Collins, 8, and Cook, 10, were last seen around midday Friday, riding their bicycles in downtown Evansdale. Police found their bicycles on Friday and a bag they were carrying on a trail near Meyers Lake. Crews used boats to search the lake, and volunteers looked in the woods Saturday and Sunday, but didn't find any sign of the girls.

Police have entered the fourth day of searching, but still have no idea what happened. About 1,000 volunteers combed a 12-square mile area over the weekend in the town of about 5,000 people.

The family of the two girls is determined to find them, even as they struggle to make it through the "fourth-day nightmare," with no clue as to what may have happened.

"We just ask that they continue to keep looking, not to give up," Collins' mother Heather Collins told ABC News today. "We will find these girls. These girls will be found."

Heather Collins said the family has strict rules with Elizabeth about how far she is allowed to go and how often she has to check in.

"If she goes more than one block, she tells us and she calls us when she gets there and she calls us when she comes home," Collins said. "This is not like her normal thing."

Heather Collins reported the girls missing about three hours after they left to ride their bikes.

"I just had a gut feeling my daughter would not leave that long," Heather Collins said. "She knows she'd be in trouble. ... That's not like her."

Collins and her husband Drew Collins said their 8-year-old daughter loves shopping trips with her mom, the color pink and her stuffed animals.

"She has a great smile. She has his sense of humor," Heather Collins said, referring to her husband. "She loves to snuggle."

"She's very girly, but also rugged," Drew Collins said. "[She] likes sports -- hockey and baseball and playing basketball."

Heather Collins said that Elizabeth's cat that sleeps with her every night has been waiting on Elizabeth's bed ever since she disappeared.

"The cat's depressed and everybody else is too," she said. Collins said that her three other children, who are 4, 6, 12, miss their sister and have been "very upset."

Heather Collins had a message for whoever took her daughter and niece.

"I want to say to them that my husband and I have forgiven them," she said. "God has forgiven them already. So we have forgiven them and we just want our children back safe and sound. That's all we want. We don't even want to know who the person is."

When asked how they keep their minds from imagining the worst, Drew Collins replied, "You don't. It's impossible. I've run over everything in my mind a thousand times and it's impossible. You just try to hold onto whatever kind of hope you have."